You have to slowly ship petrol; that puts a severe damper on the market reacting to supply and demand. Imagine how different it would be if a gas station could instantly buy out the stock of a competitor across town charging three cents less.

Anyway I fully understand the bank being slightly off the middle, but with that big of a spread the bank is either lying or has money bonfires somewhere in back. And I'm not really joking there, look at how much is spent on horrible legacy systems out of fear that replacements will have bugs... or at least different bugs.